Such apparatus are known, for example, from the German utility model GM 74 19 633. Described therein is a closed-end tube which encloses a thermocouple, in a protective structure of refractory metal oxide and graphite. Between the closed-end tube and the protective structure a ring slot is formed. The thermocouple, which is arranged in the closed-end tube, is made of expensive materials such as platinum, for use with the high temperatures existing in molten metal. Especially with continuous temperature measurements, which are necessary for constant control of the molten metal, the thermocouple is not fully protected inside the described apparatus, especially since reactive or corrosive gases enter through the structure which surround the thermocouple and can thereby destroy the structures and the thermocouple. Therefore multiple replacements of the thermocouple can become necessary. Under the influence of the high temperatures in the molten metals, silicon monoxide and carbon monoxide, for example, are formed within the protective structure of the apparatus, and these permeate the tube of aluminum oxide which surrounds the thermocouple. Hence, the carbon monoxide increases the porosity of this tube, whereby the tube is slowly destroyed. The silicon monoxide reacts with the thermocouple wire, so that this is destroyed and the temperature reading is faulty or can no longer be determined.
An additional apparatus of similar means is known from GB-B 2 193 375. This apparatus has a thermocouple with a closed end which is enclosed by an impermeable molybdenum tube coated with ceramic. This molybdenum tube is very expensive and is complicated to manufacture. In order to avoid destruction through thermomechanical stresses, the molybdenum tube is encased with several ceramic layers, which have an outwardly decreasing amount of molybdenum. These layers serve, among other things, to compensate for the temperature gradient along the molybdenum tube. These ceramic layers and the molybdenum tube can indeed also be destroyed by reactive gases, such as carbon monoxide or silicon monoxide.